One of the principal advantages of an optical waveguide configuration as contrasted to bulk crystals is that much higher intensity electric fields may be produced by a given applied voltage with the optical waveguide configuration and much lower capacitive values may be realized. Both of these operative characteristics are necessary to achieve high speed operation of such electrooptic modulators.
A thin film waveguide electrooptic modulator can operate employing any of several modulating mechanisms, e.g., Mach-Zehnder interferometry, directional coupling, or rotation of the optical polarization.
For a Mach-Zehnder interferometry type of electrooptical modulator, an optical beam is guided into a linear thin film waveguide and split into two arms, one of which is sandwiched between a pair of electrodes, and subsequently the arms are recombined into a single output beam. A phase shift between the light guided in the two different arms occurs when a voltage is applied to the electrodes and creates a change in the index of refraction in one of the arms due to either the Pockels or Kerr effect. The modulation of the output occurs since the beams in the two arms either add or cancel when they recombine depending on the phase relationship between them. The device requires a single mode linear waveguide for beam splitting and recombination.
For the directional coupling type of modulator, the optical beam is coupled into one of two adjacent linear waveguides and coupled out from the other guide. The amount of optical power which is transferred from one guide to the other guide depends on the index of refraction of the medium between the channels. By applying an electric field and altering the index of refraction between the channels, the power transferred, and hence output from either guide, can be modulated.
The modulating mechanism for the polarization type of modulator is the phase shift between the transverse electric (TE) and transverse magnetic (TM) modes in the same waveguide due to an electric field applied parallel or perpendicular to the surface of the waveguide which creates a directional change in the index of refraction in the waveguide due to a Pockels or Kerr nonlinear optical effect.
For a low voltage operating electrooptic modulator, highly responsive nonlinear optical media are required. LiNbO.sub.3 has been an important inorganic species for waveguide electrooptic modulator construction. However, there are certain inherent disadvantages in the use of LiNbO.sub.3 or other inorganic compound in an electrooptic modulator, such as the limitation of the input optical power due to the inherent photorefractive effect, and the high fabrication cost for a LiNbO.sub.3 high quality crystal.
It is known that organic and polymeric materials with large delocalized .pi.-electron systems can exhibit nonlinear optical response, which in many cases is a much larger response than by inorganic substrates.
In addition, the properties of organic and polymeric materials can be varied to optimize other desirable properties, such as mechanical and thermoxidative stability and high laser damage threshold, while preserving the electronic interactions responsible for nonlinear optical effects.
Of particular importance for conjugated organic systems is the fact that the origin of the nonlinear effects is the polarization of the r-electron cloud as opposed to displacement or rearrangement of nuclear coordinates found in inorganic materials.
Nonlinear optical properties of organic and polymeric materials was the subject of a symposium sponsored by the ACS division of Polymer Chemistry at the 18th meeting of the American Chemical Society, September 1982. Papers presented at the meeting are published in ACS Symposium Series 233, American Chemical Society, Washington, D. C. 1983.
Organic nonlinear optical media in the form of transparent thin substrates are described in U.S. Pat. Nos. 4,536,450; 4,605,869; 4,607,095; 4,615, 962; 4,624,872; and references cited therein.
J. Opt. Soc. Am. B, 4, No. 6, 968 (1987) by K. D. Singer et al describes second order orientationally ordered materials, and the relationship between molecular and macroscopic properties. Second harmonic generation measurements were conducted on poled molecule-doped polymer films of Disperse Red 1 azo dye dissolved in poly(methyl methacrylate). Electrooptic properties of poled polymer glasses were determined from the dependence of the optical phase on voltage in a Mach-Zehnder interferometer.
"Nonlinear Optical Properties Of Organic Molecules And Crystals", Volume 1, Chapter 11-1 (Academic Press, 1987), by J. Zyss et al describes a theory of TE and TM propagation in birefringent organic media which are in the form of waveguides.
Also of background interest with respect to the present invention are references which describe polymers exhibiting piezoelectric properties, such as U.S. Pat. Nos. 3,665,206; 3,694,055; 3,878,274; 4,241,128; 4,298,719; 4,356,424; and 4,751,139; incorporated herein by reference.
There is continuing research effort to develop new organic nonlinear optical media and electrooptic devices adapted for laser modulation, information control in optical circuitry, and the like. The potential utility of organic materials with large second order and third order nonlinearities for very high frequency application contrasts with the bandwidth limitations of conventional inorganic electrooptic materials.
Accordingly, it is an object of this invention to provide novel polymers which exhibit a high level of nonlinear optical response.
It is another object of this invention to provide a nonlinear optical film medium consisting of a copolymer of a vinyl halide monomer and a vinyl monomer with a pendant side chain exhibiting second order nonlinear optical susceptibility .beta..
It is a further object of this invention to provide an optical light switch or light modulator device with a polymeric nonlinear optical component consisting of a transparent film of a thermoplastic polymer having a crystallographic molecular alignment in the film plane and a net dipolar molecular orientation normal to the film plane.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples